Crack Paths 2012

initiation and early growth of cracks impossible. The current authors continue this work

by introducing “Murakami type” drilled holes [1] and FIB milled semielliptical notches

to simulate a case, where fatigue relevant inclusions hit the specimen surface. In this

case the paths and growth of microcracks can be monitored.

E X P E R I M E N T A L

Different batches of two steel types have been studied. In this paper we report results

for a bearing steel of type 100Cr6 quenched and 180 ºC tempered to hardness H R C61

(1630 M P atensile strength) and a quenched and tempered 34CrNiMo6steel with 1065

M P aand 1180 M P a yield and tensile strengths respectively. The 100Cr6 specimens

(Fig. 1a) were taken from ¼ depth of a 70 m mwrought bar. The 34CrNiMo6

specimens according to Figure 1b were machined from the centreline of 35 m m

wrought bars.

Figure 1. Dimensions of the test bars: a) for smooth bars only; b) also for drilled holes.

Axial fatigue tests for smooth and polished specimens were performed at resonant

frequencies below 200 Hz. The fatigue life target area for unnotched specimens was in

the U H C Fregion (106 < Nf < 109). Run-out specimens were broken at increased stress

amplitudes and every fracture surface was studied. The statistics of fatigue relevant

inclusions in the 100Cr6 steel could be accurately studied, as 94 % (80/83) of failures

initiated from subsurface inclusions. But in the Q T steel 34CrNiMo640% of the

failures initiated at and 20%immediately below the surface from smaller inclusions or

unidentified origins.

Artificial initiation sites were next introduced to Q T steel specimens. Fatigue

initiators were manufactured by drilling ( 5 0 ma)nd FIB milling (a = 10 … 5 0 m )and

the specimens were fatigued at the range of smooth specimen fatigue limit. These small

holes were optically monitored and video recorded for crack initiation, growth and

arrest in real time. The holes were later carefully examined with S E Mand optical

microscopes. Furthermore, cross sections of the cracks were milled with FIB and the

crack paths within the microstructure were studied using ion channeling.

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